This invention relates to an alloy for the storage of hydrogen, and more particularly to a novel and useful Mischmetal type multi-element alloy for the storage of hydrogen, which is capable of occluding a large volume of hydrogen in the form of a hydride and releasing the hydrogen readily and rapidly by application of a small amount of heat.
As a new energy source to take the place of fossil fuels, hydrogen has recently come to attract increasing attention because of its salient advantage that it has promise of limitless supply, it is clean, it is transportable and its use as an energy source does not disturb natural cycles.
Heretofore, hydrogen has been stored in the form of gaseous hydrogen or liquefied hydrogen or metal hydride. Of the various forms of storage, the storage of hydrogen in the form of a metal hydride has recently come to attract particular attention in view of its safety and in due consideration of the fact that the metal hydride reversibly permits occlusion and release of hydrogen. The requirements which must be fulfilled by a substance to be used for storing hydrogen in the form of a metal hydride are (1) that the substance should be inexpensive and readily available, (2) that it should be readily activated and should possess a high capacity for occlusion of hydrogen, (3) that it should possess a moderate equilibrium dissociation pressure near room temperature, (4) that it should exhibit a constant dissociation pressure relative within a certain range of the hydrogen/metal atom ratio and (5) that the reactions of occlusion and release of hydrogen occur reversibly at a high rate of speed.
The transition metals such as Ti, Zr, La and Mg which have heretofore been known to produce hydrides have poor qualities for use as substances for storage of hydrogen because the hydrides of these metals are highly stable thermally and do not liberate hydrogen unless their temperatures are elevated to levels higher than 300.degree. C., for example. In recent years, alloys of Ti-Ni, Ti-Co, Ti-Fe, La-Ni, Mg-Ni, Mm (Mischmetal)-Ni and Mm-Co have been developed. They, however, all have defects as substances for storage of hydrogen. Of the aforementioned alloys, those of Ti, La and Mg alloys are as thermally stable as the metals Ti, La and Mg and require a long time for effecting occlusion and release of hydrogen. The activation effected on these alloys can hardly be called easy. In the case that one of these alloys is used for the storage of hydrogen, the metals from which the alloy is produced must be of very high purity and, in this respect, there is an economic problem. Moreover since their capacities for occlusion of hydrogen are affected by the purity of hydrogen, the hydrogen subjected to occlusion by the alloys is required to possess a high purity.
Comparison of the Mm-Ni and Mm-Co alloys reveals that while the former alloy possesses a high equilibrium dissociation pressure despite a large capacity for hydrogen occlusion, the latter alloy suffers from a small capacity for hydrogen occlusion despite a low equilibrium dissociation pressure. The activation of the Mm-Ni type alloy requires the hydrogen pressure to be as high as 80 to 90 kg/cm.sup.2 or the treatment of activation to be performed for an excessively long period or to be repeated a number of times. This alloy consumes much time in occluding or releasing hydrogen and suffers the problem that the dissociation pressure is greatly affected within a certain range of hydrogen/metal atom ratio.
As an improvement over these alloys, there has been invented an alloy of the composition MmNi.sub.5-x Co.sub.x (U.S. Pat. No. 4,147,536). This alloy approximates the Mm-Co alloy in equilibrium dissociation pressure and the Mm-Ni alloy in capacity for hydrogen occlusion. U.S. Application Ser. No. 18,941 (dated Mar. 31, 1978) U.S. Pat. No. 4,222,770 covers a still better alloy for the storage of hydrogen. This alloy is represented by the general formula MmNi.sub.5-x A.sub.x (wherein, A is one member selected from the group consisting of B, Ca, Cr, Cu, Fe, V, Zn and Si). This alloy excels above all the alloys mentioned above in most properties indispensable to the storage of hydrogen such as, for example, speed of hydrogen occlusion, activation and equilibrium dissociation pressure. From the practical point of view, however, an alloy possessing much better properties is in demand.